3 Answers
3

These words have these plurals because they are loan words from Latin. Words that come from Latin that end in -um usually have plurals in -a, while those that end in -us have plurals in -i. This way of forming plurals is normal in Latin, and learned English preserves the native Latin plurals.

It comes from people who still remember that a word is a loan word and the lending language was inflected. Often people attempting to inflect the way Latin does do a poor job of it, so outside of the most common Latinisms, it would be better style to use ordinary plurals.

These words are loan words from Latin. The plurals associated with words ending in -um or -us are not dictated by practice, but by precise, Latin, rules.

In Latin - which is an inflected language - there are 5 declensions. Nouns are distributed among declensions and follow declension-specific rules.

So, a noun belonging to the second declension and ending in -us (such as lupus), will have lupi as plural, while one belonging to the same declension and ending in -um will have an -a plural (bellum -> bella).

Note that in Latin nouns have a gender, so lupus is male, while bellum is neuter.

A noun belonging to the fourth declension such as spiritus (male) will have spiritus as plural.

I wouldn't say the plurals are dictated by precise rules. On the contrary, I would say they are dictated by inconsistently applied rules that were, at some stage, informed by Latin pluralization. We have Latin words that have regular English plurals (campus pluralizes as campuses), Latinesque plurals for words people think are Latin that aren't, like octopus-octopi (octopus is Greek), words that were originally plural in Latin that are now mass nouns (data), and words where the Latin plural form is often used for singular and plural (alumni).
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KosmonautApr 13 '11 at 22:19

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@Kosmonaut -- just my opinion, but anyone who uses "alumni" as a singular should be shot into space...
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MalvolioApr 13 '11 at 23:38

The precise rules statement referred to the Latin plurals, not to the English ones :)
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Roberto AloiApr 14 '11 at 14:06

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@Malvolio If you want to extend that to Italian, anyone who uses panini as a singular word should be shot.
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Roberto AloiApr 14 '11 at 14:07

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people say "a panini"? OK, I've never heard that but yes, anyone saying it will be condemned to live on Wonder Bread and yellow mustard for the rest of his life. How do people feel about 'an agenda'?
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MalvolioApr 14 '11 at 14:31